The invention refers generally to a method for coating a substrate with a transparent conductive oxide layer, in the following designated as TCO layer, as well as a thin-film solar cell.
The coating takes place on a substrate using physical vapor deposition (PVD method), usually through sputtering in a in-line method at which the substrate is moved through a coating chamber, and is then coated. For the sputtering, the substrate is moved passed a cathode assembly as coating source which has a target of the material that is to be deposited. Into the coating chamber, a working gas is introduced which has added a small amount of hydrogen or other gases in comparison to the working gas.
Such conductive and transparent TCO layers can be used in various applications and in particular in the UV and in the IR range because of their optical properties, e.g. as a transparent electrode in thin-film solar cells or in flat screens, as a blocking layer in a selective layer system for glass, or as a IR reflection layer. In respect to these different applications, various other substrates can also be considered, e.g. glass, silicon or metallic substrates.
It is known, to manufacture transparent conductive oxide layers from various metal oxide layers (Transparent Conducting (Metal) Oxide—TCO) which due to their doping with material from the third group of the Periodic Table of Elements, e.g. aluminium, indium, gallium or boron have the required conductivity. Also through dopings with flourine, yittrium or magnesium, the desired electrical properties are attained. Well known are for instance doped layers of indium oxide, tin oxide or indium tin oxide (ITO) wherein layers of tin oxide become more important as they are manufactured significantly cheaper, are non-toxic, easy to dope, and durable within a hydrogenous atmosphere.
The coating takes place in a vacuum coater which dependent on the layers or layer systems that are to be deposited have one or more coating chambers. Serving as the target is a ceramic metal oxide target which is doped to adjust the low specific surface resistance of the layer as described for both the optical and the electrical properties required, e.g. the doping takes place by enhancement of the target material with aluminium oxide in the single-digit percents by weight. Such a layer material is known as ZnO:Al or AZO. Also, reactive or partly reactive sputtering techniques can be used.
Besides the target material, the sputtering atmosphere and other process parameters such as pressure, also partial pressure of various components of the working gas, and temperature in particular the substrate temperature, or power input also have an impact on the transparency and the specific surface resistance of the deposited layer.
It is therefore described that through the introduction of hydrogen into the argon atmosphere of the coating chamber to produce an argon-hydrogen plasma, the conductivity of the layer and the reflection in the IR range is increased with a continuously good transmission in the visible range. However with adding the hydrogen, a regular decrease of the deposition rate is observed which presents itself as a disadvantage for an efficient coating on a large scale in continuous operation systems. Additionally, the realization of the high frequency magnetron sputter (RF sputtering) of doped ZnO causes problems in regards to stability of the process, and thus, the layer homogeneity and in regards to the deposition rate. Also, the coating using direct current voltage sputtering, i.e. by means of a pulsed input of the electrical power in the cathode assembly with frequencies in the range of 3 to 50 MHz is used.
The substrate temperature is adjusted by a plane-like heating of the substrate before the coating procedure by taking into consideration the energy input during the coating. The substrate is mostly heated to 100° C. plus, wherein only with significantly higher temperatures of approximately 200-300° C., acceptable surface resistance values of the deposited layer are attained.
In particular through the temperature loading during the coating process, an outward diffusion of material from the substrate in the to be deposited or already deposited layer is however supported. The deposition of a barrier layer is a common method to avoid a contamination of the active layers through material diffusing from the substrate outwards.
For the use in thin-film solar cells, in which the TCO layer is used as a transparent, electrical surface contact which is passed by the incident light, besides a high transparency and a good electrical conductivity of the contact layer, good diffusing properties are required. If this surface contact is arranged on the side of the incident light of a solar cell, a high light incidence is possible on the one hand, and on the other hand, a certain extent of light scattering in the layer is beneficial to improve the light coupling in the absorbing semiconductor. Through multiple scattering it shall be achieved that the incident light in the layer system travels as long a distance as possible to increase the absorption proportion in the photoactive layer. In particular, total reflections of the incident light are to be inhibited. For a better coupling of the light in the layer system, the TCO layer at its borders toward the photoactive layer is also provided with a roughened surface.
To achieve exactly that, a sequence of various layers is regularly deposited. It is known in particular for the manufacturing of a roughened by PVD generated, following the deposition at first smooth, TCO layer is subsequently roughened using an etching process, however without changing the transmission properties or the electrical conductivity significantly.